1. Field of the Invention
This invention relates to magnetic resonance (MR) imaging and more particularly to a method of reducing magnetic field variations when a subject is in an imaging magnetic field.
2. Description of the Related Art
in magnetic resonance (MR) imaging it is very important to provide a homogeneous magnetic field over an imaging volume of the subject in order to acquire accurate, non-distorted images of the subject. One such way of providing homogeneous magnetic fields is to apply a static magnetic Field with a main magnet, B.sub.o, and augment the main magnetic fields with adjustment magnetic fields from auxiliary coils. These coils have shapes and current paths which add or subtract from the main magnetic field. These coils are commonly referred to as "shim" coils. The process of correcting the magnetic field inhomogeneities by passing a currents through various shim coils is known as "shimming" the magnet. The problem is, however, that these coils, and the magnetic fields produced by them, have complicated geometries, and interact with each other as well as the main magnetic field.
Measurement of field inhomogeneities when monitoring only a single resonance frequency, such as the hydrogen nuclei of water, is described in U.S. Pat. No. 4,740,753 "Magnet Shimming Using Information Derived From Chemical Shift Imaging" by Gary H. Glover, and Grant T. Gullberg, issued Apr. 26, 1988 assigned to the present assignee and hereby incorporated by reference. This patent describes shimming a magnet by identifying a matrix of magnetic field variations over space known as an inhomogeneity map.
The magnetic fields produced by an incremental current run through each of the individual shim coils are determined. Each magnetic field map is fit to a three dimensional polynomial, such as a Legendere polynomial. Next a matrix of three dimensional Legendre polynomial coefficients, describing the magnetic field produced by each of the shim coils is constructed. This matrix is termed a shim coil calibration matrix. An inverse matrix is constructed from the coil calibration matrix. The inverse matrix is matched to the measured inhomogeneity map to correct for the lowest orders of magnetic field inhomogeneity. This method ('753 patent) did not, however take into account the effects of the subject being imaged. It used a phantom with one chemical species for shimming.
The magnetic field in the imaging volume should be shimmed taking into account the effects of the subject. U.S. Pat. No. 4,987,371 by Gary H. Glover, Erika Schneider "Method for In-Vivo Shimming", issued Jan. 22, 1991 described a method of shimming the magnets taking into account the effect of the subject within the is magnet. This method, however, only corrects the lower order Legendre polynomials, and does so in Cartesian coordinates. Since each term of the polynomials corrected for provides a more accurate estimation of the magnetic field it would be advantageous to correct for higher order terms also. The '371 patent corrected for the two lowest order terms.
A similar patent, U.S. Pat. No. 5,168,232 by Gary H. Glover, Erika Schneider "Method for Rapid Magnet Shimming", issued Dec. 1, 1992, also assigned to the present assignee and incorporated by reference, describes a faster method of measuring the magnet field variation and thus decreases the time required to shim. This method ('232 patent) corrects the lowest order Legendre polynomials only for a phantom composed of a single chemical species, and not for in-vivo shimming. It does not take into account the effects of the subject on the magnetic field.
Currently there is a need for a fast method of shimming an MR magnet, taking into account effects of the subject on the magnetic field with more than one chemical species, and which corrects for higher order magnetic field inhomogeneities.